Flares give away where you are as well as let you see what's out there. They are a last resort when you are all but certain that the enemy is in front of you. Setting there in the dark you can hope that you go un-noticed. That you will see them before they find you and roll in that grenade.
But when someone fires a flare all bets are off. They now know where you are in a general area. The light of the flare also lights up the smoke trail from the launcher which is a white finger marking the launch site.
Your fear is that they are hiding out there just beyond the area that the flare lights up. That they see you and your friends but you can't see them. And when the flare dies it's darker than before, because the night vision you spent the hours before getting, was destroyed by the flare's light.
The sound of the flare and those deep dark shadows brought back those fears. Instinct says get ready, while the brain tries to say it's just TV.